User Interface (UI) is the visual part of a website, application, or hardware that determines how the user interacts with it. Designing UI involves:

1. Determining the required functionalities.
2. Understanding the user’s needs and preferences.
3. Good information architecture.
4. Building a wireframe design.
5. Building a prototype.
6. A/B testing to refine the design elements.

In eCommerce, good UI reduces bounce rates and increases click-through and conversion rates by optimizing:

• input controls (e.g., checkboxes, dropdown lists/buttons, toggles, text fields)
• navigational components (e.g., search field, breadcrumbs, pagination, sliders, image carousels)
• informational components (e.g., notifications, progress bars, tooltips, message boxes, pop-ups).

UI best practices include well-positioned search bars, eye-catching call-to-action buttons, user-friendly navigation, smooth checkouts, and F-pattern placement (positioning the most important information on the left side).